NAS326 Volume down

recently i noticed that the NAS is no longer responsive.

A reset helped but now there is no volume left. I only have one harddrive with 3TB.

I can see the disk on the WEB UI but there´s no volume.

SMART infos can be read, all looks fine.

Is there a way to save the data?

I tried to get the following information:

BusyBox v1.19.4 (2021-04-01 09:56:40 CST) built-in shell (ash)

Enter 'help' for a list of built-in commands.

~ # cat /proc/partitions

major minor  #blocks  name

   7        0     144384 loop0

  31        0       2048 mtdblock0

  31        1       2048 mtdblock1

  31        2      10240 mtdblock2

  31        3      15360 mtdblock3

  31        4     108544 mtdblock4

  31        5      15360 mtdblock5

  31        6     108544 mtdblock6

   8        0    4044975 sda

~ # cat /proc/mdstat

Personalities : [linear] [raid0] [raid1] [raid6] [raid5] [raid4]

unused devices: <none>

~ # cat /proc/mounts

rootfs / rootfs rw 0 0

/proc /proc proc rw,relatime 0 0

/sys /sys sysfs rw,relatime 0 0

devpts /dev/pts devpts rw,relatime,mode=600 0 0

ubi4:ubi_rootfs1 /firmware/mnt/nand ubifs ro,relatime 0 0

none /firmware/mnt/sysdisk ramfs ro,relatime 0 0

/dev/loop0 /ram_bin ext2 ro,relatime 0 0

/dev/loop0 /usr ext2 ro,relatime 0 0

/dev/loop0 /lib/security ext2 ro,relatime 0 0

/dev/loop0 /lib/modules ext2 ro,relatime 0 0

/dev/loop0 /lib/locale ext2 ro,relatime 0 0

/dev/ram0 /tmp/tmpfs tmpfs rw,relatime,size=5120k 0 0

/dev/ram0 /usr/local/etc tmpfs rw,relatime,size=5120k 0 0

ubi2:ubi_config /etc/zyxel ubifs rw,relatime 0 0

configfs /sys/kernel/config configfs rw,relatime 0 0

~ # su

BusyBox v1.19.4 (2021-04-01 09:56:40 CST) built-in shell (ash)

Enter 'help' for a list of built-in commands.

~ # mdadm --examine /dev/sd[abcd]3

mdadm: cannot open /dev/sda3: No such device or address

mdadm: cannot open /dev/sdb3: No such device or address

mdadm: cannot open /dev/sdc3: No such device or address

mdadm: cannot open /dev/sdd3: No such device or address